1. Field of the Invention
The present invention relates to a color thermal printer in which heat from a thermal print head is used to transfer ink from an ink transfer medium to a print medium such as paper. More particularly, it relates to a color thermal printer in which color printing is effected by reciprocally moving the paper within a prescribed range.
2. Description of the Prior Art
In a color thermal printer, a print medium such as paper, for example, together with an ink transfer medium (hereinafter referred to as "ink donor film") are transported through the space between a thermal print head and a platen roller while heat is applied by the thermal print head to effect printing by transferring ink from the ink donor film to the paper.
FIGS. 5 and 6 show examples of the arrangement of conventional color thermal printers. With reference first to FIG. 5, which shows a color thermal printer that employs a reciprocating paper transport system, printing paper 10 is pinched between two sets of pinch rollers 12 and 14 and is moved backwards and forwards by the rotation of these pinch rollers. Between the pinch rollers 12 and 14 is a transfer section constituted by a gap between a platen roller 16 and a thermal print-head 18 through which the paper 10 passes. Under the paper 10 as it passes through the transfer section is an ink donor film 20 which is moved in unison with the movement of the paper 10 by the rotation of film rollers 22 and 24. Heat from the thermal print-head 18 is used to selectively transfer ink from the ink donor film 20 to the paper 10 located between the platen roller 16 and the thermal print head 18.
FIG. 6 illustrates a color thermal printer in which the paper 10 is moved in only one direction by a rotational mechanism. Here the platen roller is constituted by a platen drum 28 which is provided with a gripper 30 for clamping the paper 10 to the surface of the platen drum 28. Film rollers 22 and 24 are to transport the ink donor film 20.
The operation of the printer arrangements shown in FIGS. 5 and 6 will now be described. With reference first to FIG. 5, the paper 10 is moved by the rotation of the pinch rollers 12 around the platen roller 16 and through the transfer section between the thermal print head 18 and the platen roller 16. The transport of the paper 10 is continued to bring the leading edge of the paper through the second set of pinch rollers 14. During this part of the operation the thermal print head 18 is retracted away from the platen roller 16 and the ink donor film 20 is wound on until it is in the prescribed position.
Next, the thermal print head 18 is held against the platen roller 16 at a prescribed pressure and the thermal transfer printing process begins. During the thermal printing process the paper 10 is supported by the frictional force of the pinch rollers 12 and is printed as it is transported in the direction indicated by the arrow 100 by the rotation of the pinch rollers 14 and the platen roller 16. The rotation of the pinch rollers 12 and 14 is synchronized with the rotation of the platen roller 16 to keep the paper 10 smooth and taut, and the ink donor film 20 is wound onto the film roller 22 at a rate which matches the transport speed of the paper 10. At the point at which the transfer of a color is completed, the trailing edge of the paper 10 has to stay held between the pinch rollers 12.
To print the next color, the paper 10 has to be moved back to a predetermined print start position. This comprises retracting the thermal print head 18 away from the platen roller 16 and moving the ink donor film 20 to a prescribed position at which the ink of the desired color is located. During this operation the paper 10 is wound back to the print start position in the direction indicated by the arrow 200, by the reverse rotation of the pinch rollers 12 and 14. At the print start position the leading edge of the paper 10 remains held between the pinch rollers 14.
Each color is thus thermally transferred, i.e. printed, by the above sequence of operations, so completion of a color printing is effected by repeating this sequence of operations a number of times which corresponds to the number of colors which have to be transfer-printed. A three-color printing of yellow, magenta and cyan, for example, would be accomplished by repeating the above sequence of operations three times.
Next, with reference to the operation of the conventional apparatus illustrated by FIG. 6, with the paper 10 held in position on the platen drum 28 by the gripper 30 the platen drum 28 is rotated until the print start position is reached, during which time the thermal print head 18 is maintained in a position of retraction from the platen drum 28 while the ink donor film 20 is moved on to bring it to the necessary position for the start of the transfer printing. After the rotation of the platen drum 28 is thus stopped at the print start position, the thermal print head 18 is brought into contact with the platen drum 28 at a prescribed pressure and the ink transfer begins. The printing of a color is completed when the platen drum 28 has described one revolution in the direction indicated by the arrow 300, following which the next color is transferred by another revolution of the platen drum 28 in the direction indicated by the arrow 300, starting from the print start position. Thus, a three-color transfer printing operation involves three such revolutions by the platen drum.
The conventional color thermal printers described above have the following drawbacks. In the case of the reciprocating system illustrated in FIG. 5, the paper 10 is supported and transported by the frictional force of each of the two sets of pinch rollers 12 and 14. To ensure that the paper 10 is maintained in a state of tension during its transportation, the pressure and frictional force of each of the sets of pinch rollers 12 and 14 have to be maintained at a constant level, for which a separate means of adjustment is needed. Also, the paper 10 has to be positioned precisely to achieve good registration of the second and subsequent colors, but with these conventional color thermal printers the detection and alignment of the paper 10 needed for such precise positioning are difficult.
In addition, for the paper 10 to be moved reciprocally while being printed with a plurality of colors, the paper 10 has to be held throughout by the pinch rollers 12 and 14. In turn, this means that the length of the paper 10 that can be printed by the thermal print head 18 is limited to the length of the paper 10 minus the length of the portions from each of the sets of pinch rollers 12 and 14 to the platen roller 16.
The unidirectional transport system used in the color thermal printer shown in FIG. 6 is arranged so that the transfer of each color is effected with one revolution of the platen drum 28, which resolves the positional alignment problem of the apparatus of FIG. 5. On the other hand, however, the diameter of the platen drum 28 has to be set according to the length of the paper 10, so any increase in the length of the paper 10 has to be matched by a corresponding increase in the diameter of the drum 28, and hence in the drum's radius of curvature. This makes the transport mechanism larger, while the thermal print head also has to be larger to match the larger drum. Thus, the end result is an increase in the cost and size of the apparatus.
Moreover, increasing the radius of curvature of the platen drum 28 also increases the area of contact between the thermal print head 18 and the platen drum 28, spreading the applied contact pressure over a larger area. As a result, the thermal print head 18 has to be pressed against the platen drum 28 with a very strong force in order to produce good-quality printing, and extended use of the thermal print head 18 at these higher pressures making the heating elements of the print head more prone to damage and shortens the life of the print-head.